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\section*{Acknowledgments}
\label{sec:acknowledgments}
We would like to thank Marten van Dijk for suggesting the ``sparse'' prefix tree approach and Madars Virza for productive conversations that helped steer this work.
We also thank the anonymous reviewers for their useful feedback.
This research was supported in part from USA NSF under CNS grants 1413920, 1718782, 1514261, 1652259, by DARPA \& SPAWAR under grant N66001-15-C-4066, by HK RGC under grant ECS-26208318, and by a NIST grant.